Method of reducing metallic corrosion by fuming nitric acid by addition of fluoride ion to acid



United States Patent METHOD OF REDUCWG METALLIC CORROSION 'BY FG NITRIC ACID BY ADDITION OF FLUORIDE ION TO ACID Eric Ran, Toms River, N. L, assignor to the United States of America as represented by the Secretary of the Navy No Drawing. Application July 8, 1955 Serial No. 520,934

4 Claims. (Cl. 23-157) (Granted under Title 35, U. S. Code (1952), sec. 266) I The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to a method for supressing the corrosion of metals by nitric acid.

More specifically, the invention is directed to the prevention of corrosion of stainless steel in solutions of white fuming nitric acid.

The storage of white fuming nitric acid in No. 347 AISI stainless steel drums and other containers leads to the deterioration of this acid within a few months. This deterioration is caused by the surface attack of the acid on the container and the accompanying corrosion of the metals involved.

It is, therefore, the main object of the present invention to provide a process whereby the corrosion of stainless steel in solutions of white fuming nitric acid is substantially eliminated.

It is another object of this invention to provide a process for chemically treating a metal in order to render said metal suitable for use as a container for fuming nitric acid.

It is a further object of this invention to inhibit the corrosion of stainless steel in a relatively simple, yet effective manner.

Briefly, the present invention involves coating stainless steel with a metal fluoride surface layer.

A metal fluoride coating may be obtained by pre-treating stainless steel with hydrofluoric acid. Pre-treatment tests were made by immersing stainless steel strips in 46% hydrofluoric acid for various lengths of time to form a metal fluoride film on said strips. After drying, the strips were immersed in white fuming nitric acid. Initially, corrosion of the treated strips was much less than that of untreated stainless steel; however, after 200 hours in white fuming nitric acid, the treated strips began corroding at a rate identical with that of the untreated stainless steel strips. This indicated that the fluoride coating had been removed. In order to maintain the fluoride coating, it was found that a trace of hydrofluoric acid or other suitable fluoride must be added to the white fuming nitric acid to thereby provide for the establishment of a mobile equilibrium between fluoride ion in white fuming acid solution, and fluoride deposition on the metal surface. In eflect, such a procedure would provide for the continual replenishment of fluoride layer upon the stainless steel surface. An addition of 1% of a 46% aqueous solution of hydrofluoric acid will suflice to attain the desired effect. It was also found that the addition of NaF, NH -HF or any other fluoride carrying agent soluble in fuming nitric acid to the fuming nitric acid inhibits the nitric acids corrosivity towards metals.

In a preferred process, pre-forming a fluoride film on stainless steel is not necessary. A trace of hydrofluoric 2,837,407 Patented June 3, 1958 acid or other suitable fluoride is merely added to the nitric acid which is in contact with a stainless steel container. The hydrofluoric acid initially reacts with the stainless steel to form a fluoride film on the stainless steel and subsequently provides for the establishment of a mobile equilibrium between fluoride ion in white fuming nitric acid solution, and fluoride deposition upon the metal surface.

The following is a series of tests given as examples to show the effect of fluoride coating on stainless steel.

Example I Two samples of stainless steel No. 347 were pre-treated for twenty-four hours in 46% hydrofluoric acid in order to form a fluoride layer on the samples. Then to test the resistance of the treated strips to corrosion by white fuming nitric acid, the strips were immersed for hours in white fuming nitric acid to which 1% by volume of 46% hydrofluoric acid had been added. As indicated above, hydrofluoric acid was added to the nitric acid in order to provide for the continual replenishment of the fluoride layer on the Stainless steelsurface. After 140 hours, it was found that the resulting penetration of the samples was 0.00035 cm. Two similar but untreated samples were immersed in unmodified white fuming nitric acid for 140 hours. The penetration of the untreated samples was 0.0036 cm.

Example II Two untreated samples of stainless steel No. 347 were immersed for 140 hourse in white fuming nitric acid to which 1% by volume of 46% hydrofluoric acid had been added. The penetration was 0.0001 cm.

It will be observed from the above examples that a fluoride coating on stainless steel effectively inhibits the corrosion of stainless steel by nitric acid. In effect, a one hundred-fold decrease in the rate of corrosion was made possible by the trace addition of hydrofluoric acid.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A method for inhibiting corrosion of untreated stainless steel containers by white fuming nitric acid stored therein, said method comprising adding at least 1%of an inhibitor such as one from the group consisting of hydrofluoric acid, sodium fluoride and ammonium fluoride to said acid.

2. A process according to claim 1 which comprises adding 1% of sodium fluoride to the white fuming nitric acid.

3. A process according to claim 1 which comprises adding 1% of hydrofluoric acid to the white fuming nitric acid.

4. A process according to claim 1 which comprises adding 1% of ammonium fluoride to the white fuming nitric acid.

References Cited in the file of this patent UNITED STATES PATENTS 1,549,022 Otte Aug. 11, 1925 1,553,321 Meiklejohn Sept. 15, 1925 2,357,219 Mott Aug. 29, 1944 2,390,402 Uhlig Dec. 4, 1945 2,760,845 Kanarek et al Aug. 28, 1956 2,769,690 Mahan Nov. 6, 1956 2,794,703 Kindsvater June 4, 1957 

1. A METHOD FOR INHIBITING CORROSION OF UNTREATED STAINLESS STEEL CONTAINERS BY WHITE FUMING NITRIC ACID STORED THEREIN, SAID METHOD COMPRISING ADDING AT LEAST 1% OF AN INHIBITOR SUCH AS ONE FROM THE GROUP CONSISTING OF HYDROFLUORIC ACID, SODIUM FLUROIDE AND AMMONIUM FLUORIDE TO SAID ACID. 